The present invention relates generally to devices for cutting sound grooves on a disc recording medium. More particularly, the device has excellent recording characteristics. Continuous chips, cut from the original record discs, are prevented from adhering to the groove, the cutting stylus, and other parts, since the static electricity generated during the cutting process is effectively discharged.
Sound signals are ordinarily recorded on an original record disc coated with an acetate or vinyl chloride base material, that is, a so-called "lacquer disc". Recording involves cutting a groove thereon by means of a device having a cutting stylus. As a material for this cutting stylus, sapphire has heretofore been used. However, a sapphire has a short serviceable life because it is not very hard and has a relatively rapid wear.
Accordingly, attempts have been made to use a harder diamond, in place of a sapphire, for the cutting stylus. The life of a diamond cutting stylus is undoubtedly very long, because of the hardness of diamond. However, it has been found that a diamond stylus generates more static electricity during recording than a sapphire generates, since diamond is a carbonaceous material. For a diamond stylus, different kinds of electrostatic charges accumulate on the confronting surfaces of the continuous chip and the cutting stylus and on the confronting surfaces of the chip and the groove after cutting. When such a great quantity of static electricity is generated, the resulting coulomb force causes the chip to adhere to the cutting stylus and the cut groove of the lacquer disc. It is difficult to remove this chip, even by means of a vacuum suction, which has been used heretofore.
When the chip adheres to the cutting stylus and the lacquer disc, the cutting function is impaired. For this reason, cutting styluses employing diamond tips have not been used heretofore.
Accordingly, in order to overcome the above described difficulties, we have previously described a cutting device having a grounded electroconductive material on the mirror surface of a diamond stylus, as disclosed in United States Patent Application, Ser. No. 659,027, entitled "DEVICE FOR CUTTING A SOUND GROOVE IN A DISC RECORDING MEDIUM", filed Feb. 18, 1976, now abandoned. In accordance with this previous cutting device, the chip produced at the time of cutting is guided into contact with the electroconductive material. The electrostatic charge accumulated on the chip escapes to ground by way of the electroconductive material, thereby preventing static electricity. However, this cutting device has a problem since the cutting stylus is grounded by way of the cutter head.
Accordingly, we have considered the principle by which static electricity is generated when two objects of different compositions come into mutual contact and then separate.
First, if the two objects contact each other a, transfer of charge carriers (which may be considered to be electrons or ions) occurs in conformance with the difference in the interfacial energies at the mutually contacting surfaces. As a result, an electrostatic potential difference is produced between the two objects. This electrostatic potential difference causes the charge carriers to travel between the two objects, without quantum dynamical energy loss. Charges of different kind and equal quantity accumulate, respectively, on the two objects. This transfer of the charges reaches an equilibrium state and thus end, after a time, in conformance with the properties of the objects.
If the two objects in mutual contact are separated, there is a reverse flow of one portion of the charge carriers, which have accumulated in conformance with the above mentioned electrostatic potential difference. A remainder portion of the charge carriers remains on their respective object. Furthermore, charge carriers may also escape by being discharged into the air through the objects since there is a rise in the potential on the charged parts, which rise accompanies the decrease in electrostatic capacitance which occurs when the objects separate.
Next to be considered is the generation of static electricity which occurs when an original record disc (lacquer disc) is cut by a diamond cutting stylus. This cutting of the lacquer disc may be regarded as a phenomenon wherein the contacting and separating of the chip cut from the lacquer disc and the cutting stylus is continuously repeated. The chip and the stylus have an electrostatic potential difference. Here, there is a great difference of the interfacial energy levels between the nitroacetate lacquer disc and the diamond cutting stylus. For this reason, a relatively large charge of static electricity is generated and stored on both the chip and the cutting stylus.
Furthermore, the quantity of frictional charge generated between two electrical insulators is proportional to the length of frictional movement. Since the chip slides a relatively long distance along the mirror surface of the cutting stylus, an even greater quantity of static electricity is generated on the chip, the cutting stylus, and related parts. For this reason, the chip produced by the cutting operation adheres to the cut groove and the cutting stylus. As a result, a good recording cannot be accomplished.
In view of the above considerations, we have reasoned that, in order to suppress the quantity of static electricity, generated at the time of cutting, on the chip and the diamond cutting stylus, the chip should slide over the shortest possible distance while in contact with the surface of the diamond cutting stylus. Moreover, the chip should be contacted and guided by a structure made of a material having an interfacial energy level which differs by a very small amount the energy level of the chip. That is, it should be a material which does not readily generate static electricity when the chip slides over it.